Cylinder has spark but not firing..

OK if the plastic bag on the dipstick tube is pulled in or if your vacuum gauge is reading vacuum not pressure you have a intake sealing problem, this is very common way of testing had to do it every time we tuned up a old FE type ford engine 352.390.428 50% of the time the gaskets were sucked into runners, also carbs were blamed for being bad no adjustable idle on cylinders sharing runners (we had a vacuum gauge so didn't need plastic bag)